1. Field of the Invention
The present invention relates to tyres for motor vehicles, in particular for lorries and heavy truck vehicles; even more specifically the invention relates to tyres with a directional tractive tread pattern, i.e. tyres suitable for the driving wheels on the rear axle of a lorry, for example of the type which may be used for travelling long distances on motorways. The tyres to be mounted on the driving axles normally have a tread with a pattern comprising a plurality of circumferential grooves intersected by transverse grooves so as to form a plurality of blocks, whence the name of xe2x80x9cblockxe2x80x9d tyres as opposed to xe2x80x9cribbedxe2x80x9d tyres formed by continuous circumferential ribs having inferior tractional properties.
2. Description of the Related Art
The tyres for the driving axles of lorries must satisfy numerous requirements.
In fact, the tyres must have good tractional power both on dry and on wet grounds and in particular on grounds covered with fresh or compacted snow.
Moreover, there is an increasing demand from the market and on the part of various legislations for low-noise tyres; the use of xe2x80x9cblockxe2x80x9d tyres, however, generally results in a greater amount of noise than that produced by xe2x80x9cribbedxe2x80x9d tyres.
The tread, moreover, must ensure good wear uniformity together with a good performance in terms of number of kilometers travelled.
According to the present state of the art, tyres which are able to deal with the abovementioned problems and offer a solution thereto already exist.
For example, the patent IT 1,245,773 in the name of the Applicants describes a tread comprising at least four rows of blocks which are delimited by at least three circumferential grooves of different width, the two narrowest of which being arranged laterally, on opposite sides of the equatorial plane of the tyre, in an axially external position, and by a plurality of inclined transverse grooves connecting pairs of adjacent circumferential grooves. In this solution, the narrow groove which separates the two lateral rows of blocks on each shoulder of the tyre is characterized by having a width not greater than 2.5 mm and a depth not greater than that of the converging transverse grooves, while the blocks of the two lateral rows are each other circumferentially staggered so that each block of each row is located alongside two consecutive blocks of the adjacent row.
Said patent describes other solutions comprising for istance two axially internal circumferential grooves delimiting a circumferential rib on the sides of which transverse grooves are formed which are similarly inclined with respect to the equatorial plane which they meet, so as to give the rib the appearance of two adjacent rows of blocks.
In effect, the rib is formed by two rows of blocks which are separated by a very narrow central groove that is aligned with the equatorial plane.
In a further embodiment the two rows of blocks of the central rib may be joined together so as to form a single row passing through the equatorial plane and, according to a further embodiment, the blocks of the central rib may be configured with substantially hexagonal shapes.
A further example of a directional tractive tyre is described in the patent application 97IT-MI00103A in the name of the same Applicants.
Said tyre has a tread comprising on both sides of the equatorial plane of the tyre a central row and an intermediate row of blocks between a central longitudinal groove formed astride the equatorial plane and a pair of longitudinal grooves.
The blocks of the central row are adjacent and not circumferentially staggered in relation to the blocks of the intermediate row.
Moreover, the blocks of the central row are separated from the blocks of the intermediate row by narrow longitudinal grooves defined as xe2x80x9csipesxe2x80x9d which have a width lower than 1 mm.
Said longitudinal sipes extend circumferentially in a zigzag pattern so as to favour interlocking of axially adjacent blocks and reduce the mobility of the blocks themselves in the longitudinal direction.
The adjacent blocks of the central and intermediate rows are circumferentially separated by transverse sipes alternating with transverse grooves parallel to the transverse sipes and in turn comprising, from the bottom, further sipes.
The depth of the transverse grooves in the central rows ranges between 1 and 4 mm and in the intermediate rows between 5 and 10 mm.
Furthermore, the tread comprises shoulder blocks which are located alongside two adjacent blocks of the intermediate rows.
Said embodiment, owing to the transverse grooves which are provided at the bottom with sipes, allows suitable tractional features to be obtained owing to the presence of said transverse grooves and to a good rolling resistance which can be essentially correlated to the generally reduced mobility of the tread blocks under the tyre footprint, due to pressing of adjacent blocks against one another as a result of closing of the sipes.
Applicants"" goal is to realize a tyre for lorries having a tread pattern of directional tractive type which represents an alternative to that of the cited known art, improving where possible some performance features thereof, in particular as regards silent travel and tractional power, without affecting the properties of stability.
Thus, starting from the abovementioned tread described in patent application 97IT-MI00103A, it has been developed a tread still provided with longitudinal sipes separating the central and intermediate rows arranged on either side of the equatorial plane.
In fact the small width of these longitudinal sipes, resulting in the mutual approach of central and intermediate blocks in the presence of transverse forces, allows the features of a high-rigidity rubber block to be maintained.
Therefore, it was possible to ensure transverse stability in the severest travel conditions for a lorry, for example when negotiating fast bends.
However, contrary to the teaching coming from said patent, it has been thought to circumferentially stagger with respect to one another both the blocks of each central row in relation to those of the intermediate row and the blocks of the two central rows on each side of the equatorial plane.
It is in fact considered that the simultaneous presence of said two circumferential staggered arrangements of the blocks would result in a reduction in the noisiness during the impact of the tread on the ground.
It has also been thought to achieve optimum properties of tractional power by adopting between the rows of central and lateral blocks a plurality of transverse grooves of considerable depth, i.e. with values which are substantially equal to the thickness of the tread.
It has also been thought to improve the tractional features by inclining the transverse grooves with respect to a plane perpendicular to the equatorial plane; this inclination has been achieved by making the centre lines of the transverse grooves converge in the direction of forward travel along a plane parallel to the equatorial plane.
According to a first aspect thereof, the invention relates to a tyre for vehicles, comprising:
a carcass structure including a central peripheral portion and two side-walls terminating in a pair of beads for fixing a wheel to a rim;
a belt structure coaxially associated with the carcass structure;
a tread with a predetermined thickness between the external surface and an internal surface in contact with a thin elastomeric sheet for attachment to the belt.
The tread coaxially extends around the belt structure and comprises a row of central blocks and a row of intermediate blocks, both the rows being arranged on each side of the equatorial plane of the tyre between a central longitudinal groove formed astride the equatorial plane and a pair of longitudinal lateral grooves; the blocks of the central and intermediate rows are circumferentially spaced by a plurality of first and respectively second transverse grooves extending in a direction substantially perpendicular to a predetermined direction of forward travel of the tyre, each block being formed by a pair of transverse sides, respectively a front side and rear side relative to said direction of travel, and by a pair of longitudinal sides.
The blocks of the central rows are separated from the blocks of the intermediate rows by a pair of circumferential sipes.
The tyre is characterized in that:
a) the blocks of the intermediate rows are circumferentially staggered by a first predetermined quantity Q relative to the blocks of the central rows;
b) the blocks of the central rows arranged on either side of the equatorial plane are each other circumferentially staggered by a second predetermined quantity Qxe2x80x2;
c) the first and the second transverse grooves have centre lines converging in the direction of forward travel D on planes parallel to the equatorial plane;
d) the first and the second transverse grooves have centre lines inclined by an angle xcex1 in opposite directions to one another with respect to said planes parallel to the equatorial plane;
e) the depth of the transverse grooves is equal to at least 95% of the thickness of said tread.
In a preferred embodiment, on either side of the equatorial plane, in a position axially outside the intermediate rows, the tyre comprises a row of shoulder blocks and elastic means for connecting adjacent blocks.
Conveniently said elastic connection means consist in the presence of an elastomeric relief extending from the bottom of the groove up to a predetermined height.
Advantageously, for the purposes of improved silent performance, the tyre comprises shoulder blocks which are circumferentially staggered relatively to the blocks of the intermediate rows.
Preferably the tyre is characterized in that the first quantity Q of circumferential staggering of the blocks is between 57% and 67%.of the length of a block and the second quantity Qxe2x80x2 of circumferential staggering of the blocks of the central rows is comprised between 56% and 66% of the length of a block.
Even more preferably the tyre is characterized in that the second quantity Qxe2x80x2 of circumferential staggering of the blocks of the central rows is substantially equal to the first quantity Q of staggering of the blocks of the intermediate and central rows.